DESCRIPTION: (Scanned from the applicant's description): Cerebral blood flow is of fundamental importance for normal development of the brain during fetal life. Oxygen stores of the body are minimal yet cerebral metabolic rate is high, and thus prompt adjustments of flow are necessary to match oxygen supply with oxygen need. Inadequate blood flow and oxygen delivery in the perinatal period cause tragic lifelong consequences including cerebral palsy and mental retardation. Until the present time, there has not been any method to measure cerebral blood flow in utero on a continuous basis. In the last two years, we have successfully adapted laser Doppler flowmetry to measure cerebral blood flow in the chronically prepared, near-term fetal sheep. Using the method, we have shown that nitric oxide mediates about 60 percent of the increase in flow that occurs in response to hypoxic stress. We have also obtained early evidence that the brain enters a state of regulated hypometabolism upon hypoxic stress, presumably by curtailing nonessential metabolic activities, and thereby rations oxygen use. In this reapplication, we propose six specific aims. First, we will compare the results of laser Doppler flowmetry with those obtained with fluorescent microspheres. We believe this comparison is an essential step for a complete validation of the new methodology. At the same time, we will validate a new method we have developed to measure heat production and oxygen use by the fetal brain. Second, we will continue to investigate the role of nitric oxide in mediating the increases of cerebral blood flow and reductions in metabolic rate in response to acute hypoxia. Third, we will test whether adenosine mediates the flow increases and hypometabolism that compensate during fetal hypoxia, i.e. whether it plays a role similar to that which we have already shown for nitric oxide. Fourth, we will test the importance of carbon dioxide in regulation of the fetal circulation and inquire about its interactions with nitric oxide and adenosine. Fifth, we will carry out experiments to distinguish between regulated hypometabolism of the fetal brain and the inevitable reduction of 02 use that accompanies oxygen starvation. Sixth, we will test whether the mechanisms controlling blood flow and metabolic rate in the brain are blunted after gestation at high elevations with exposure of the fetus to hypoxemia for several months. These studies will provide continuous measurements of cerebral perfusion and local heat production from a small region of the fetal brain for the first time. Our broad goal is to establish the control factors for oxygen delivery and use by the fetal brain and thereby to optimize brain development and minimize hypoxic injury.